Open Fermentation

by Jim Busch
This article is going to cover the concepts of using open fermenters in
brewing. The debates over open versus closed fermentation will no doubt
continue as long as there are interested brewers to debate. I intend to
present some of my feelings, opinions, and experiences with using open
fermenters, and point out some of the inherent pros/cons of using this
technique. I want to emphasize one thing about this issue: the choice of
fermenters is not going to be *the* deciding factor in your finished product,
many other factors will play a more important part in the character of
your beer. Namely, malt choices, mashing programs, and above all, yeast
strain/viability/cleanliness will be the dominant influences on the finished
beer. Having said this, there are instances where breweries who changed
from open fermenters to closed unitanks have noted distinct changes in the
perceived quality of the beers, when judged by experienced taste panels. [1]

Open fermentation is a concept that most homebrewers think is a sure route
to infected beer, or as something to be employed in some dark cellar in an old
European brewery. I say nonsense! Think for a minute about some of the
best world class beers and then think of how many are made using open
fermenters: Sierra Nevada, Anchor, numerous English, Belgian and yes, even
German brewers use them. It is a common sight in Bavaria to see a brewer
mucking around in the thick krausen on top of the open fermenter, collecting
samples, skimming yeast, generally doing things that homebrewers are told
to avoid. Eric Warner has noted in his excellent book on Wheat beers that
open fermenters are the preferred method of German weizen production [2],
and that when open fermenters are used the yeast can be repitched for many
more generations than when a closed fermenter is used.

So whats an open fermenter? At the simplest, it is a vessel with an
open top. Depending on the size of the fermenter, they are often covered
by some form of lid. The bigger versions are truly open, large shallow
vessels, some are lined with stainless steel or an enamel like coating that
is usually used over a concrete/block foundation. Often the fermenters are
just large stainless steel cylinders. Most, but not all, have some form
of attemperater device, to combat the temperature rise during ferments. This
can be in the form of exterior jacketing, or metal piping that is immersed
in the wort, cold water or glycol is pumped inside the pipes, cooling the
ferment. Probably the most classic open fermenters are the Yorkshire Squares
used at the Samual Smiths brewery in Tadcaster, England. These are made of
flat slate walls, sealed together, with a collecting lid where the excess
krausen is contained.

OK, so your thinking open fermentation only works in big breweries since
they are filtering the air, and keeping the whole room under positive
pressure, and nobody is allowed in. Yes, and no. Sure, lots of breweries
go to the extreme of maintaining a separate room with filtered air. Lots
more don't do anything. Certainly, the breweries in England that I visited
never went to the extreme of filtered air, nor did the breweries in Bavaria
and Belgium. Belgian methods of brewing may seem strange , but the
dominant flavor profiles found in Belgium beers are a result of the choice
of a yeast strain(s) that throws high levels of esters and phenolics, and
rarely a result of some infection in the fermenter (even though this is
the way to produce lambics, the word infection is a misnomer in
this context). Certainly, the Bavarian brewmasters would recoil in horror
if any foreign bacteria or wild yeast were to be found in the open
fermenter, and in practice, they are not a problem.

I did not always use open fermenters, the first hundred or so of
my beers were made with a "closed carboy" system. I put closed in quotes
since the carboy can be fitted with a blowoff tube, resulting in a kind
of hybrid closed/open fermenter. Since fall '92, I have been using a
open fermenter exclusively, and I am a devoted fan of the concept. My
fermenter is a stainless steel cylinder, of roughly equal height to width,
with a heavy lid. If you brew with a 10 or 15 gallon stainless steel kettle,
this can double as your fermenter, once you remove the hot break. Some
brewers employ modified 1/2 BBl Sankey kegs, and these too make excellent
open fermenters. I have also read of brewers modifying Golden Gate kegs and
using these as fermenters. The least desirable, but easiest to start with,
is the plain plastic bucket. The reason I say least desirable is that
cleaning plastic is more difficult than stainless, and the inevitable
scratches in the plastic walls can be harder to sanitize. Even so, I know
of an award winning homebrewer who ferments in food grade plastic trash
cans, and another 2 BBl brewpub who ferments in large High Density Poly-
Ethelyne (HDPE) containers. I have found that as you increase the brew
length (volume of beer produced), it is easier to fabricate some sort of
fermenter that can hold the entire batch. In this way, you will be limiting
the number of vessels to sanitize and clean up. It is far cheaper and easier
to fabricate or modify a container to be an open ferementer than to make a
closed one, particularly as the volume increases. An important consideration
when sizing the fermenter is to account for a large amount of krausen that
can develop during the ferment. Head space of 30% is optimum, but less can
be used, with the result being some possible loss of product (which also
occurs when using the blowoff carboy method).

Of course, there are some limitations to using open fermenters. I believe
they are no more prone to infections than using carboys, but there is an
increased chance for infection if one has numerous fruit flies or other
animals around the fermenter, provided the lid is off. Probably the
biggest limitation is that of time, I do not advise leaving the beer in
the fermenter for more than 2 weeks. Of course, any ferment should be
racked by the second week, so maybe this isn't such a limitation after all.
The reason time is more important in open fermenters is not so much the
proximity of the still beer to dead yeast, but of the danger of oxidation
reactions occurring as the beer sits. In a closed system, this will not
be a problem, but as long as the beer is moved in a timely manner, the
CO2 produced during open fermentation will protect the beer. Another
important factor to consider is the overall cleanliness of the fermentation
area. It need not be sterile, but a reasonable degree of cleanliness is
in order, in particular for fermentation inside of a refrigerator. Many
brewers use a temperature control device to moderate the ferment temperature
inside of a refrigerator. If you use an open fermenter inside of a
refrigerator, be sure to clean all obvious sources of contamination and
general dirt. Some may even want to sponge down the interior of the
refrigerator with a mild sanitizer such as chlorine/water. At the very
least, all spilled trub, yeast and wort should be thoroughly cleaned up.
Household pets should also be prevented from crawling into the fermenting
beer, they may like the results too much! My fermenter is located in the
basement, a few feet off the ground, away from large drafts and any foreign
debris sources.

Heres a summary of how I use my open fermenter. Since I use a stainless
fermenter, I don't want to use a chlorine based sanitizer, due to problems
with corrosion. So, I prepare a solution of Iodophor, at 12.5 ppm (1 oz
in 10 gallons), of a few gallons. Using rubber gloves, I sponge the
sanitizer over the sides of the fermenter. I let it run out the drain,
then back over the sides of the fermenter. I also run Iodophor through
my wort chiller into the fermenter, followed by a hot water rinse. Once
the hot water is drained, the vessel is ready for cast out wort. I fill
the fermenter from the wort chiller, oxygenate and add thick yeast slurry.
As in any fermentation, there is no substitute for pitching enough viable
clean yeast. The key to success with an open fermenter (or closed) is
a sanitized vessel, and an adequate amount of pitching yeast. Remember to
use significantly more yeast if the original gravity of the wort is higher
than 1.060. If one is using enough yeast, visible fermentation is evident
within 12 hours (ale wort, fermented between 60-70 F). As soon as the
fermenter is full and the yeast is pitched, place the lid on. Once the
fermentation is generating a thick head of krausen, I have found it helpful
to leave the lid partially cracked, allowing an airspace for the large
amounts of CO2 to vent. With the ferment in high krausen, the classic
dense rocky heads will form. At this stage, trub will be scrubbed from
the ferment, and rise to the surface, along with other solid matter that
was carried over into the fermenter. This scum can be skimmed off with a
sanitized spoon (I leave a long handled stainless steel spoon in some
Iodophor and just rinse it off when needed). The ability to skim the
trub and yeast that rise to the top of the fermenter is one of the
main advantages of open fermentation. Don't overdo it, but about once a
day or every other day, depending on the rate of ferment, skim the top.
Many ale yeasts tend to flocculate at the top of the ferment as the ferment
diminishes. This yeast is excellent to skim and store in a sanitized
container, in a cold fridge (as close to 32F as possible). When choosing
yeast to save, be sure to wait a few days into the ferment so that the
trub is scrubbed away and the harvested yeast is clean. As the ferment
dies down, keep the lid over the vessel. Another great plus of open
fermenting is the ease of dry hopping. What I do is let the main fermentation
subside and when the yeast clumps to the surface, skim as much off as
possible, then add the loose whole hops (I find that whole hops give better
aroma and are easier to use with an open fermenter). Allow at least 3
days time for the dry hopping to take affect. I would avoid leaving the
beer in the primary for longer than 2 weeks, and aim for 10 days when
dry hopping, and a mere 5 days otherwise. These are optimum figures for
ale ferments, and are often not realistic in homebrewing, the primary
cause being inadequate oxygenation of the cast out wort, and/or insufficient
yeast cell densities/viabilities in the pitching yeast. To rack off of
the hops, use a sanitized copper/brass or stainless "choreboy" scouring
pad, held over the racking cane with a rubber band. Alternatively, the
hops can be removed with a sanitized strainer, provided a minimum of air
is introduced to the still beer.

Important points to remember:

Pitch plenty of healthy yeast slurry, between 1/2 and 1 oz. of slurry per
gallon of wort, or at least 1 QT of yeast starter per 5 gallons wort.
Professionally, pitching rates are on the order of 1/2 to 1 pound of
slurry per barrel of wort.

If you have a way to increase the dissolved oxygen levels of the wort, do
so. At the least, splash the wort when filling the fermenter. Using an
airstone and filtered air for the first few hours is even better.

Keep the lid on most of the time. This helps to trap the CO2, and since
the finished still beer will have around 1 volume of dissolved CO2 in solution,
there will be a thin blanket of CO2 over the beer, protecting the beer
from detrimental oxidation reactions. Once the ferment is producing large
quantities of foam/krausen, it is good to leave the lid cracked, but as
soon as the krausen begins to subside, keep it covered.

To harvest yeast, wait for the initial fermentation to scrub the dark
trub and remove this trub with a sanitized spoon prior to harvesting the
yeast. Some yeasts can be stored for 2-3 weeks in a sanitized container,
preferably in a cold 32F refrigerator. Some yeasts are quite prone to
mutations, and if this is the case, storing for 10 days is the upper limit.
If the yeast is to be stored for longer periods, it is advisable to feed
fresh wort into the container, and allow another period of fermentation
to occur prior to pitching into a fresh batch of wort.

Fining agents, such as isinglass, can be added directly to the primary
fermenter, provided the desired degree of attenuation has been achieved.
Polyclar, or PVPP can also be added directly to the fermenter, although
these products should be filtered from the beer prior to serving.

Lagers can be made using open fermenters but the timing and temperature
control issues make it more difficult to do than ales. When fermenting
lagers in a refrigerator, I would recommend keeping the lid on for the
entire primary, and racking the beer into the lagering vessel as soon as
primary fermentation is complete, or 90% complete.

Conclusion:

Fermenting in an open vessal can be an effective and convienent method of
beer production and yeast harvesting, especially when brewing ales. It is
an easy way to skim trub that rises to the surface of the ferment, and
can be a cost effective method to increase batch sizes.

Footnotes:

Jim Busch is an electrical engineer developing real time systems for NASA.
He has been an all grain brewer and beer traveler since 1988. When he is
not beer hunting, he can be found on the Internet at:
busch@daacdev1.stx.com